1. Field of the Invention
The present invention relates to a battery back-up system. In particular, the invention relates to such a system for vehicles powered by internal combustion engines, including hybrid vehicles.
2. Prior Art
The need of vehicles powered by internal combustion engines for reserve electric-energy-storage capacity has been recognized practically as long as such vehicles have been in existence. Initially, the need was felt on cold mornings when the vehicle's stock battery proved insufficient to start its engine. Later on, the need became more general, as vehicles were loaded with peripheral equipment requiring electrical energy and presenting the potential for draining the vehicle battery to the point where it was incapable of starting the engine. This problem arose as soon as vehicles were equipped with such peripherals as lights that could be operated with the engine off, but has become more acute in recent decades as the peripheral equipment added to vehicles began to include sophisticated technical apparatus that require significant amounts of power, such as, for example, electrical wheelchair lifts and related equipment. With vehicles so equipped, the concern arises that the single storage battery will run down to the point where it can neither operate the wheelchair lift or start the engine. This is of particular concern, because it is possible that an operator can enter his or her vehicle, only to become stranded (trapped) there without enough battery power to either start the vehicle or operate the wheelchair lift.
The solution of simply adding a second battery to the vehicle has been around for a long time, particularly in regions that experience extreme cold. By connecting two 12-volt batteries in parallel, one can usually ensure adequate energy availability even when low temperatures have reduced the effectiveness of the batteries. A variant on this solution is to carry to the vehicle a supplemental battery that will supply the electric energy needed to start the engine when the on-board battery has become incapable of doing so because of malfunction, because peripheral equipment was left on and drained the battery, or because the air temperature is −40°. In this latter approach, the supplemental battery is temporarily connected in parallel with the on-board battery and the engine started. With the engine started, the supplemental battery is disconnected, and the car's engine-driven generator (“alternator” in its modern embodiment) eventually brings the on-board battery up to its fully charged state. It is possible to remove the supplemental battery even before the vehicle's own battery is fully charged because, once the engine is operating, the alternator not only provides all of the power needed for the devices associated with the vehicle, but has enough left over to charge the on-board battery. It is for this reason that the on-board battery is sometimes referred to as the “starter battery,” since that is usually all that it is needed for, apart from providing low levels of electric power for the radio and other minor peripheral equipment. With the engine operating, the alternator supplies all the car's electrical needs: the electronic ignition system; the lights; the radio; the GPS; the seat heaters; etc.
For definitiveness, the remainder of the discussion of the prior art will refer to a single supplementary or auxiliary battery. It is to be understood, however, that the comments may be equally well directed toward a setup in which multiple batteries serve as supplementary batteries.
Presently, the reliability of the auxliary battery is maintained by maintaining it as a separate entity that is not hard-wired into the vehicle electrical system. The disadvantage of this method is that one has to maintain the auxiliary battery in a charged state, shuttle it back and forth between the vehicle and its storage location, and then hook it up properly to the starter battery.
If all one wished to do was to double the battery capacity, one would simply hardwire the two batteries in parallel. In that case, they would also be in parallel with the alternator, which would charge them together in the same manner and at the same time that the present single on-board battery is charged. This would, however, defeat the purpose of the dual battery as a back-up system. To serve that purpose, the two batteries must be able to be isolated from one another at certain times.
Another problem with having two batteries that are isolated from one another is that there will regularly be times at which one or the other battery will be significantly discharged with respect to the other. Specifically, a fully charged “12-volt” lead storage battery will develop approximately 13.6 volts across its terminals but, when fully discharged, will present a voltage of 10 volts or even lower. Under those circumtances, connecting the batteries in parallel will lead to an extremely high current from the charged battery to the discharged one, current that is wasteful, shortens battery life, and is even potentially dangerous. The high current arises from the fact that by design starter batteries have a very low output impedance, which is desired and needed in normal vehicle operation. The conventional low-impedance starter battery is capable of providing very high current when called on to do so when cranking the engine. Conversely, it is also capable of receiving a charge very rapidly, indeed, as fast as the alternator can charge it.
What is needed then is a battery back-up system that permits the incorporation into a motor vehicle of a plurality of batteries, each battery capable of being used independently of the other(s), in such a manner that each battery is maintained and recharged by a single conventional alternator. What is further needed is such a battery back-up system that maximizes the time that both batteries are at or near full charge, whereby at least one of the batteries is maintained at or near full charge at all times. What is yet further needed is such a system that requires no intervention by the vehicle operator, yet nevertheless provides a means for the operator to override the system during exigent circumstances. Finally, what is needed is such a system that can be readily installed in vehicles already on the road, as well as in new vehicles.